Image intensifier devices are used to amplify low intensity light or convert non-visible light into readily viewable images. Image intensifier devices are particularly useful for providing images from infrared light and have many industrial and military applications. For example, image intensifier tubes are used for enhancing the night vision of aviators, for photographing astronomical bodies and for providing night vision to sufferers of retinitis pigmentosa (night blindness).
Image intensifier tubes are well known and used throughout many industries. Conventionally, image intensifier tubes are identified by the generic generation of their design. In the prior art, there have evolved four conventional generations of image intensifier tubes. Currently in the prior art, image intensifier tubes range from Generation O to the current state of the art Generation III (GEN III) image intensifier tube. As the technology of image intensifier tubes has developed, each successive generation has embodied advantages in performance over the previous generation.
Referring to FIG. 1, a current state of the prior art Generation III (GEN III) image intensifier tube 10 is shown. Examples of the use such a GEN III image intensifier tube in the prior art are exemplified in U.S. Pat. No. 5,029,963 to Naselli, et al., entitled REPLACEMENT DEVICE FOR A DRIVER's VIEWER and U.S. Pat. No. 5,084,780 to Phillips, entitled TELESCOPIC SIGHT FOR DAYLIGHT VIEWING. The GEN III image intensifier tube 10 shown, and in both cited references, is of the type currently manufactured by ITT Corporation, the assignee herein. In the shown GEN III tube 10, infrared energy impinges upon a photocathode 12. The photocathode 12 is comprised of a glass faceplate 14 coated on one side with a antireflection layer 16, a gallium aluminum arsenide (GaAlAs) window layer 17 and gallium arsenide (GaAs) active layer 18. Infrared energy is absorbed in GaAs active layer 18 thereby resulting in the generation of electron/hole pairs. The produced electrons are then emitted into the vacuum housing 22 through a negative electron affinity (NEA) coating 20 present on the GaAs active layer 18.
A microchannel plate (MCP) 24 is positioned within the vacuum housing 22, adjacent the NEA coating 20 of the photocathode 12. Conventionally, the MCP 24 is made of glass having a conductive input surface 26 and a conductive output surface 28. Once electrons exit the photocathode 12, the electrons are accelerated toward the input surface 26 of the MCP 24 by a difference in potential between the input surface 26 and the photocathode 12 of approximately 800 volts. As the electrons bombard the input surface 26 of the MCP 24, secondary electrons are generated within the MCP 24. The MCP 24 may generate several hundred electrons for each electron entering the input surface 26. The MCP 24 is subjected to a difference in potential between the input surface 26 and the output surface 28 which is typically about 900 volts, whereby the potential difference enables electron multiplication.
As the multiplied electrons exit the MCP 24, the electrons are accelerated through the vacuum housing 22 toward the phosphor screen 30 by the difference in potential between the phosphor screen 30 and the output surface 28 of approximately 6000 volts. As the electrons impinge upon the phosphor screen 30, many photons are produced per electron. The photons create the output image for the GEN III image intensifier tube on the output surface of the optical inverter element 31.
The primary performance limiting component of the GEN III device is the MCP 24. The MCP 24 limits the resolution of the tube, has poor noise characteristics, and is both difficult and expensive to manufacture and test. Furthermore, the MCP 24 requires an expensive power supply, shortens tube life by outgassing and provides poor high-light resolution. Since the MCP 24 is glass, outgassing of water occurs over time which degrades the integrity of the environment within the vacuum housing 22. The outgassing mechanism is somewhat overcome by the ion barrier added to the MCP 24, however the presence of the ion barrier reduces the signal to noise ratio (SNR) of the tube. The resolution of the viewed image is lowered by the Nyquist limit of the MCP 24 attributed to the discrete sampling nature of its design, and the fact that the electrons emitted by the MCP 24 have a large radial velocity component which defocuses the image formed on the output screen. The glass material of the microchannel plate 24 also tends to have poor secondary emission characteristics which create a poor SNR. Furthermore, the MCP 24 requires large voltage differentials between its input surface 26 and its output surface 28 and between the photocathode 12 and the input surface 26 to adequately drive the electrons. The power supply needed to maintain such varied electric potentials increases the cost and complexity of the GEN III image intensifier tube 10 and tends to have a low reliability as a result of its complexity.
In the photocathode 12 of the GEN III image intensifier tube 10, there is no applied bias. The operation of the photocathode 10 relies on diffusion of electrons from the GaAs active layer 18 to the negative electron affinity (NEA) coating 20. As such, the GaAs active layer 18 has a doping level which is a compromise between high diffusion length (i.e., low doping), to promote energy absorption, and high emission probability (i.e., high doping), to promote electron emission probability. Since both the photon absorbing and electron emitting characteristics of the photocathode 12 are controlled by the single GaAs active layer 18, the GaAs active layer 18 cannot be doped to a concentration that optimizes either the photon absorbing or electron emitting characteristics, thereby detracting from the operation of both.
In an attempt to avoid the disadvantages associated with conventional MCP's in image intensifier tubes, the prior art proposed an entirely solid state image intensifier device. In the proposed solid state image intensifier device, an avalanche photodiode (APD) is used to multiply electrons from the photocathode in place of the MCP. An example of an avalanche photodiode is shown in U.S. Pat. No. 5,146,296 to Huth, entitled DEVICES FOR DETECTING AND/OR IMAGING SINGLE PHOTOELECTRON. The output image is provided by light emitting diodes (LED's) coupled to the avalanche photodiodes. Between the avalanche photodiodes and the LED's is positioned a light absorbing or reflecting layer to prevent optical feedback. Thus, the device represents an all-solid-state image intensifier. The proposed solid state image intensifier is very difficult to manufacture with gain and noise characteristics approaching those of the GEN III tube. One of the primary problems is the LED output devices, which at best has 10% electron-to-photon conversion efficiency. Not only is it difficult to manufacture the LED to this efficiency, but now the avalanche photodiode (APD) gain must duplicate the electron gain of the MCP, plus the conversion gain of the phosphor, plus an additional factor of 10 to account for the inefficiency of the LED.
Additionally, if red LED's are used, an additional gain may be required to compensate for the low efficiency of the eye in the red wavelengths. APD's capable of producing such high gains are possible, but are unacceptably noisy. Thus special low noise amplifiers must be used, and these require an unreasonably large number of stages to produce sufficient gain to match the GEN III device. The low-noise amplifier stages also have a very exacting structure and are difficult to manufacture. Furthermore, the LED's must be isolated from one another to force the light out and to prevent light-piping sideways through the LED layer. The isolation, etching and contacting of the LED's complicates the processing, and the resulting pixelized output may degrade the image.
Therefore, there exists a need in the art of image intensifier devices for an image intensifier that performs as well, or better then, the GEN III image intensifier tube and avoids the use of a microchannel plate and LED's.
It is therefore a primary objective of the present invention to provide an image intensifier tube that utilizes a solid state electron amplifier to provide an image upon a phosphor screen within a vacuum housing while providing state of the art performance.